Refrigeration is generated in sub-ambient temperature apparatus such as air separation plants and liquefiers to produce the necessary sub-ambient temperatures required for the operation of such apparatus. The refrigeration is generated by expanding a compressed stream to a lower pressure with the performance of work to generate a cold exhaust stream that is used to impart the refrigeration into the apparatus. The work of expansion must be extracted from the apparatus and this is done through the generation of heat in an oil brake mechanism that dissipates outside the apparatus, through the load applied by a compressor or by an electrical generator to generate electricity that can be sold to the electrical power grid to offset power costs in running the plant.
For example, in an air separation plant air is compressed and purified to produce a compressed stream. Part of the compressed stream is introduced into a heat exchanger and is cooled to a temperature suitable for its rectification within one or more distillation columns to produce nitrogen and possibly also, oxygen and argon product streams. Where both nitrogen and oxygen product streams are desired, the compressed and purified air is introduced into a double column unit having a high pressure distillation column to separate nitrogen from the air and thereby to produce a crude liquid oxygen column bottoms also known as kettle liquid. A stream of the bottoms liquid is further refined in the low pressure column to produce an oxygen-rich liquid column bottoms from which the oxygen product is taken and a nitrogen-rich vapor column overhead. The high pressure distillation column also produces a nitrogen-rich vapor column overhead that is at least in part condensed through indirect heat exchange with the oxygen-rich liquid column bottoms in the low pressure column to produce liquid nitrogen reflux for both the high and low pressure columns. The nitrogen product is taken from the nitrogen-rich vapor produced in the low pressure column, the high pressure column or both or also, from part of the condensed nitrogen-rich liquid.
In an air separation plant, the nitrogen-rich vapor from the low pressure column and oxygen-rich liquid are introduced along with other streams into the cold end of the main heat exchanger to help cool the air and for discharge as nitrogen and oxygen products. However, in the discharge of such products there are thermal losses at the warm end of the main heat exchanger as well as heat leakage into an insulated containment known as a cold box that is used to house the distillation columns. In order to compensate for such heat leakage and thermal losses, refrigeration is generated by further compressing another part of the compressed and purified air, partly cooling such air in the main heat exchanger and then introducing the compressed and partially cooled air into a turboexpander. The turboexpander may either be coupled to the booster compressor or may be used to drive the generator. In air separation plants that are designed to produce a high pressure gaseous oxygen product, a stream of the oxygen-rich liquid from the low pressure column is pumped and such stream is warmed within the main heat exchanger to produce the gaseous oxygen product at the high pressure. In order to warm such a stream in the main heat exchanger, a yet further part of the air is further compressed and then cooled within the main heat exchanger. Such air, after cooling, can also be introduced into a turbine known as a liquid expander to generate more refrigeration. This turbine can be coupled to an electric power generator.
In a liquefier, in which a gas is liquefied, for example, an atmospheric gas or natural gas, an incoming gas stream is compressed and cooled in a heat exchanger to a liquid or a two-phase state that is separated into a vapor that is recycled and the liquid product of the plant. Part of the compressed stream is further compressed and expanded in a turboexpander or potentially a series of tuboexpanders operating at successively lower temperature levels to produce exhaust streams that are recirculated into the heat exchanger to cool the incoming gas to be liquefied. The turboexpanders can be coupled to booster compressors, oil brake mechanisms or electrical generators. There are many different cycles employed with respect to liquefiers and the foregoing generally represents one of such cycles.
In expanders used for purposes such as discussed above, the expander has a radial inflow layout in which the incoming compressed stream is directed by nozzles to an impeller that can be coupled to a generator for generating electrical power. In situations in which the turboexpander is coupled to an electrical generator, typically, an alternating current induction motor is used as the generator. As can be appreciated, in order to supply electrical power to an electrical power grid, the electricity must be generated at slightly above the voltage in the grid to drive the generated power into the grid and at the line frequency employed in the grid, for example, 60 Hertz. By using a motor as a generator that is designed to operate at 60 Hertz, the application of such technology becomes very straight forward. One problem, however, is that the motor-generator is designed to operate at a maximum nominal 3,600 rpm based upon a line frequency of 60 Hertz and the turboexpander is designed to operate at much higher speeds, typically between 20,000 and 40,000 rpm. Hence, a complex geared transmission must be used that inherently will produce irreversible losses as heat between the turboexpander and the generator. Additionally, since the generator speed is constrained at 3,600 rpm, the turboexpander's speed is also constrained. Any turbomachinery has a specific isentropic efficiency that is related to the energy of the flow passing through such machinery and the shaft speed of the machine. More specifically, for a given turboexpander, the efficiency or in other words, the degree to which energy of the flow passing through the turboexpander will be transmitted will depend on flow rate and enthalpy drop in the flow passing through the turboexpander and speed of the impeller. Consequently, the gearing between the turboexpander and the generator is designed for a normal operational speed of the turboexpander that upon such normal operating conditions of flow, pressure and temperature of the flow, the efficiency of the turboexpander will be at a maximum. However, during turndown of an air separation plant or a liquefier or other sub-ambient temperature apparatus during which the apparatus is operating at less than standard design conditions, the turboexpander is being constrained to operate in an inefficient manner in order to provide the set speed to the geared transmission connected to the generator.
Recently, high speed motors have been developed having sophisticated electronic drive units that allow the motor to operate at any speed and specifically, in speed regimes that are the same at which a turboexpander of a sub-ambient device are operated. It has been found by the inventors herein that the sophisticated electronic drives and such high speed motors in particular, can be used in refrigeration systems that are employed in sub-ambient temperature apparatus, such as have been discussed above, in a manner that will obviate the problems in the prior art outlined above.